An electrophotographic image forming apparatus is equipped with a fixing device which fuses and then fixes unfixed toner on recording paper.
Such a fixing device frequently uses the heat roller fixing method, in which a heat roller and a pressure roller are adopted. According to a fixing device that uses the heat roller fixing method, a heating source (heater) is provided either in a heat roller or both in the heat roller and in a pressure roller. The surfaces of both of the rollers are heated by the heating source. The heat of the rollers fuses unfixed toner, and the unfixed toner thus fused is then fixed on recording paper by the contact pressure between the heat roller and the pressure roller.
In recent years, efforts have been made to provide an image forming apparatus whose processing speed (paper feeding speed) is higher than before so that the number of sheets to be printed per unit of time increases. Therefore, according to image forming apparatuses of recent years, the time (passage time) required for recording paper to pass through a fixing nip section (a section where the heat roller makes contact with the pressure roller) becomes short. As such, the contact pressure between the heat roller and the pressure roller tends to be set high so that appropriate fixing performance is ensured even when the passage time is short. It is conceivable to improve the fixing performance by increasing a fixing nip width, instead of increasing the contact pressure, so that a great amount of heat is supplied to unfixed toner. This, however, causes a problem that the recording paper acquires wrinkles because the recording paper passes through a large contact pressure area.
On the other hand, if the contact pressure between the heat roller and the pressure roller is increased, the recording paper does not properly peel off from the heat roller due to the increased adhesiveness exerted between a printed surface of the recording paper and the heat roller. This causes a JAM (a fixing JAM) to tend to take place in the fixing device.
FIG. 6 is an explanatory diagram illustrating an example of a JAM which occurs in a fixing device of the heat roller fixing method. The fixing device 200 illustrated in FIG. 6 includes a heat roller 201, a pressure roller 202, a cleaning device 203, paper peeling nails 204a and 204b, heaters 205a to 205c, a heat roller cover 206, a pressure roller cover 207 and the like.
JAM examples 1 and 2 illustrated in FIG. 6 occur in the case in which a leading end of recording paper is not properly peeled off from the heat roller 201 by the paper peeling nail 204a. When there occurs a JAM such as JAM 1 or 2 in the fixing device, the user is in need of removing the recording paper that caused the JAM. This takes a lot of time and effort. Further, in a case such as the JAM example 2, where the recording paper that caused the JAM is stuck in the heat roller cover 206, removing of the recording paper itself may be difficult. Furthermore, because the interior temperature of the fixing device is high immediately after a JAM occurs, the user needs to wait to remove the recording paper until the interior temperature of the fixing device becomes lower. This is time-consuming.
In a case where a JAM has occurred in the fixing device, it is normal that, after the user removes the recording paper that remains in the image forming apparatus, the same image that was printed on the sheet that caused the JAM is printed again (a printing retrial) on another recording paper under the same image forming conditions, under which the image was printed on the recording paper that caused the JAM (see, for example, Patent Document 1: Japanese Unexamined Patent Application Publication No. 251488/2006 (Tokukai 2006-251488; published on Sep. 21, 2006)).
FIG. 7 is a flow chart showing an example of a printing process performed by a conventional image forming apparatus. According to the example illustrated in FIG. 7, when a printing instruction (for example, a printer mode, a fax mode or a copier mode) is entered by a user (S101), an image process for printing is performed with respect to supplied image data (S102). Subsequently, the image data that has been subjected to the image process is temporarily stored in a memory (S103).
An image is printed onto a sheet in accordance with the image data that was stored in the memory in S103 (S104). During this step, it is determined based on detection whether or not a JAM has occurred during the printing process (S105). When it is determined that no JAM has occurred, it is determined whether or not there remains image data that has not been printed (S106). When it is determined that there remains such image data, the printing process of S104 is performed with respect to the image data that has not been printed. When it is determined that there remains no image data that has not been printed, the process is terminated.
When it is determined that a JAM has occurred in S105, the printing operation of the image forming apparatus is suspended (S107). Subsequently, it is displayed on a display section of the image forming apparatus where a JAM has occurred (S108). In accordance with the display, the user removes the sheet that caused the JAM and the sheet that remains in the apparatus. It is determined whether or not the sheet that caused the JAM and the paper that remains in the apparatus are removed (S109). When such sheets are removed, it is determined whether or not all attachments are attached to the image forming apparatus (whether or not each attachment is properly attached and the exterior lid is closed) (S110). When it is determined that all of the attachments are attached to the apparatus, the image forming apparatus is initialized (S111), and the printing process of S104 is performed again (reprinting process) in accordance with the image data that was stored in the memory in S103.
However, according to the conventional art, in a case of a JAM, the same image printed on the sheet that caused the JAM is printed again under the same image forming conditions, under which the image was printed on the sheet that caused the JAM. This causes a problem that recurrence of a JAM similar to the previous JAM is likely to occur.
In other words, in a case where the combination of image data and image forming conditions is likely to cause a JAM to occur in a fixing device of an image forming apparatus, it is most likely that a JAM similar to the previous JAM recurs when the printing is performed again under the same image forming conditions, under which the image was printed on the sheet that caused the JAM.